Durability assessment of low-pressure cold-sprayed TiO2 photocatalytic coatings: Photocatalytic and mechanical stability

Abstract

The effectiveness of the immobilised photocatalyst depends on several factors. However, considering photocatalyst operation, two of them are crucial: mechanical stability, which is responsible for maintaining the durability of coatings, and photocatalytic stability, which provides long-term reactivity. Only combining both factors enables proper recovery and recycling of the photocatalyst. Accounting for these features enables the longevity of the photocatalyst, which is particularly important when new types of coatings are being developed, e.g., photocatalytic coatings deposited using the low-pressure cold spray (LPCS) method. At the same time, photocatalyst durability, under operational conditions, is often overlooked by researchers. To address this issue, a sol-gel-derived TiO₂ powder is immobilised through the LPCS method to create the photocatalytic coatings, varying in thickness. In order to assess their durability, some of these samples are subjected to a humid atmosphere for 1000 h (100% humidity, 40°C), simulating potential working conditions. The mechanical stability of the TiO₂ coatings is assessed by comparing the cohesion and adhesion of the samples before and after the humidity test. Additionally, the samples are described in terms of thickness, surface roughness, structural stability, and band gap values. The photocatalytic stability is determined through a four-cycle photocatalytic decomposition of the methylene blue dye and by comparing the decomposition efficiency between as-sprayed and chamber-aged samples. Experimental results demonstrate that the TiO₂ coating's degree of crystallinity affects both mechanical and photocatalytic stability. Thicker, more crystalline coatings are more mechanically durable after ageing and are capable of retaining their photocatalytic properties over a multiple-cycle decomposition test.